The present invention relates to an isolated signal source. In particular, the present invention relates to an optically isolated current mirror.
There is a need for analog signal isolation in a wide variety of electronic applications. Historically, transformers have been used to isolate alternating current (AC) signals. For direct current (DC) applications, direct current signals have been isolated by first modulating the DC signal with an AC signal, then isolating the AC signal with a transformer and finally demodulating the DC signal from the AC signal.
The use of a transformer to perform the isolation has several disadvantages for DC applications. First, isolation transformers are relatively large and expensive. Second, modulating and then demodulating the DC signal requires complex circuitry which adds the size and expense of the isolation circuit. Finally, isolation transformers require a separate power supply for each side of the isolation barrier. This increases the power consumption of the isolation circuit.
Recently, circuit designers have started using optical devices for isolation. These designs, however, have been complex and have required a power supply on each side of the isolation barrier. Further, optical isolators have transfer ratios with relatively high temperature coefficients. This reduces the accuracy of the isolated signal and the effectiveness of the overall isolation circuit.